Most tyre shops make their money on fitting rather than the tyres themselves. A tyre which sells for £100 may make about £15-20 of profit. It really depends where you shop.
A mate works at Kwik Fit and recently I was there with another mate having some part worn Eagle F1s fitted on his Lobster Claws.
A merc came in with a flat tyre and they sold him a budget 195/55/15 for something like £95 fitted.. that's going to be a good £40 of profit there. Where as my local tyre shop would be more like £70 so making less money
Especially on big brand tyres you don't make as much money as you may think..